An ad-hoc wireless network is a collection of wireless nodes that can communicate with each other without the aid of any pre-existing communication infrastructure. The medium access control protocol plays a crucial role in such networks by enabling efficient communication among multiple devices on the shared medium. At high data rates, busy-tone protocols will be much more efficient than collision avoidance protocols because of transceiver turn-around times. Wireless collision detect (WCD) is proposed to enhance the performance of existing busy-tone solutions. This improves the performance and eliminates hidden and exposed nodes. This protocol is based on collision detection by the destination node (DN) and performs similarly to the Ethernet protocol. We analyze the protocol and get very close agreement between analytical and simulation results. At 100 Mbps, WCD achieves 77% efficiency even with packets as short as 192 bytes which is a 10 fold improvement over the IEEE 802.11 MAC protocol. When the time during which the node is vulnerable to a collision is three times the time to detect a carrier, WCD performs 20% better than RI-BTMA.

The medium used by indoor wireless LANs is a shared, scarce resource. Hence, efficient use of this medium is important, which in turn requires an efficient MAC protocol. Moreover, next generation wireless LANs will have to support data rates from 10 to 100 Mbits/s. At these data rates, turnaround times in half-duplex radios are a large overhead. This makes the current wireless standards inefficient. However, busy-tone solutions can get around this problem. In spite of their better performance, the busy-tone protocols are not widely used because of the hardware cost, i.e., the requirement of two transceivers-one each for the data and feedback channel. A novel wireless transceiver architecture which mitigates this hardware cost by overlaying the data channel and the feedback channel in the same frequency band is designed. It is shown that this transceiver can be built with current technology. Based on this wireless transceiver, a new MAC protocol called wireless collision detect (WCD) is proposed. This protocol uses a carrier detect signal to decrease the collision probability and receiver initiated feedback to handle hidden nodes. We derive a mathematical expression for the throughput of the protocol. The simulation results match the analysis. At 100 Mbits/s, WCD achieves 77% efficiency with 192 byte packets. For this scenario, the throughput of the IEEE 802.11 MAC protocol and RI-BTMA is 3% and 52%, respectively.

The growing importance of Web traffic on the Internet makes it important that we have accurate traffic models in order to plan and provision. In this paper we present a Web traffic model designed to assist in the evaluation and engineering of shared communication networks. Because the model is behavioral we can extrapolate the model to assess the effect of changes in protocols, the network or user behavior. The increasing complexity of Web traffic has required that we base our model on the notion of a Web-request, rather a Web page. A Web-request results in the retrieval of information that might consist of one or more Web pages. The parameters of our model are derived from an extensive trace of Web traffic. Web-requests are identified by analyzing not just the TCP header in the trace but also the HTTP headers. The effect of Web caching is incorporated into the model. The model is evaluated by comparing independent statistics from the model and from the trace. The reasons for differences between the model and the traces are given.

Wireless technology is an attractive option to network devices in the home as it does not require rewiring the home. The media access protocol (MAC) plays a crucial role in allowing multiple devices to transmit and receive on the shared wireless medium and thereby determining the efficiency of a wireless home network (WiHN). It is found that many of the protocols proposed to date like the Distributed Foundation Wireless MAC (DFWMAC) in the IEEE 802.11 standard have poor performance at high data rates (100 Mbps). We modify the slotted-idle sense multiple access protocol and propose a simple and efficient MAC protocol which fits well with the WiHN architecture. We present a queuing analysis and simulation results of the performance of the wireless home network MAC (WiHNMAC) protocol.

Cable plants were initially designed for one-way broadcast of analog television signals (from the head-end to the neighborhood). They are now being upgraded to provide an upstream path (from the home to the head-end). New challenges arise in using the upstream channel since the available bandwidth is low and the noise levels are high. Different variations of the reservation Aloha protocol have been proposed as the MAC protocol to efficiently share the scarce upstream capacity. The efficiency of the reservation protocol highly depends on the capacity assigned to the reservation channel. In this paper we present a control mechanism that dynamically adjusts the operating parameters to the current load on the system. The control mechanism does not require any framing structure and is built around a "sea of mini-slots". The performance under both static and highly dynamic loads is close to optimum. The station implementation remains particularly simple and the downstream control structure is also simple. Results are given here for fixed length data units (ATM cells) but the algorithm extends very simply to variable length MAC frames.

The cable TV plant is a broad bandwidth channel into the home and community that has been used primarily for the distribution of television. As the plant is upgraded, there is an opportunity to use it for the transport of high-speed digital signals. While the downstream channel is relatively easy to exploit, the topology and physical characteristics of the upstream channel present new challenges at both the physical and the media access layers. We present a media access protocol that efficiently transfers data on this channel. A primary goal of the design is to keep that portion of the protocol resident in the station as simple as possible. Thus, we use centralized control located in the cable head-end. A station wishing to transmit sends a request to the head-end using a contention channel. The head-end acknowledges the request and then schedules the request, informing the station, by means of a grant message, exactly when to transmit. The protocol performs efficiently under a wide range of conditions, and is robust in the presence of channel errors.

A revolution is occurring in the scope and range of information, communication and education services that will be made available to schools, libraries, town-halls, clinics and, most importantly, residences. These services will be provided initially, primarily over hybrid fiber-cable systems, either by telephone companies or cable companies. The old cable plant is being upgraded and used in totally new ways. The topology and physical characteristics of the upstream channel present new challenges for efficient channel access. We present a media access protocol that efficiently transfers data on this channel. A primary goal in the design was to keep the portion of the protocol resident in the station as simple as possible. Thus we use centralized control located in the cable head-end and minimize intelligence in the station. We refer to this protocol as centralized priority reservation or CPR. A station wishing to transmit sends a request to the head-end using a contention channel. The head-end acknowledges the request and then schedules the request, informing the station by means of a grant message when to transmit. The protocol performs well under heavy load. Performance is affected little by the number of stations, the speed of the system and the physical length of the system.

As LANs/MANs operate at higher speeds (100 Mb/s-Gb/s) and over longer distances (10's - 100's km) the access mechanism has to be chosen carefully so as to provide high utilization. Protocols are available that operate efficiently at high speeds and long distances. We describe here a particularly simple access mechanism referred to as Simple (S). Performance is simulated under a range of traffic, physical lengths, number of stations and overload conditions. S is compared with both Fasnet and the Distributed Queue Dual Bus (DQDB) schemes. It is almost as efficient as more complex protocols and its overload performance is well behaved.

An access protocol is described that is particularly appropriate for high-speed local and metropolitan area networks. It permits efficient use of the shared transmission medium, in particular where the electrical length of the medium may be many times the duration of an average packet, as well as sharing of the medium in an almost arbitrary manner among a number of competing traffic types or user classes. Each station independently measures the traffic flow on the medium and adjusts its own transmission rate according to the measured rate. The scheduling algorithm has been tested on slotted unidirectional buses and rings. By simulation the performance of the algorithm is compared to other efficient algorithms. The algorithm can control the admitted traffic of a particular type to within 2% (of full load) of the target value.>

Summary form only given, as follows. Our three publications - Communications Magazine, IEEE Transactions on Communications, and the Journal on Selected Areas in Communications - cover an area of technology that is surely one of the most exciting and exuberant today. Electronic communications makes headlines in the daily press, creates whole new industries, and has an ever-increasing effect on everyone's life. The technical details captured in our pages are, or at least should be, closely tied to the strong currents of technological change which are sweeping through society. In order to capture thjs spirit of newness and excitement, our publications need to serve our readers by staking out claims in the new areas and publishing more in those areas. Personal computer communications, the role of electronic countermeasures in recent international events, the revolution in device fabrication for communications applications, new optical fiber communications systems, and the new products and services brought on by communication deregulation are all "hot" topics which should be given more emphasis, and will be. We would welcome your additions to this list. On the other hand, there are constraints on this process. The Communications Society spends more money on its publications than on anything else. Thus, it is unavoidable that we publish less on subjects which are old and overworked and do not represent the future. Our finite page limits compel us to be selective and to emphasize papers which are both of high quality and in the ground-breaking subject areas. The three of us are asking our editors and reviewers to impose a stricter standard of significance on papers so as to emphasize those that make a sizeable increment in the state of the art or wnich favor new topics over old ones. Higher rejection ratios may be expected in some areas. We want to hear from you how this is working, because feedback from readers is the most desirable and reliable guide for our editorial policies. Please do not expect any large immediate changes in the content of the publications. Yet, we hope that within six months you will begin to find the articles ·more interesting, relevant, and original, and above all that you are actually reading more of them. This will be the real test of our new policies.

Local area networks designed to carry a variety of traffic such as data, voice, facsimile, and video should be able to implement low latency virtual circuits to meet the demands of periodic traffic. We have designed a partially distributed algorithm to efficiently schedule voice traffic on a unidirectional bus system called Fasnet. Virtual channels are allocated for the duration of a talk spurt and relinquished during the intervening silent intervals. Conversations already in progress, but without an assigned circuit, take precedence over newly arriving calls. Unused voice capacity may be utilized by data stations when required. Simulations of the system indicate that performance is close to that obtained by an ideal TASI multiplexer. While the algorithm is unfair, this is not a significant factor unless the network is loaded very heavily. Further, except for a "sojourn" time unused voice capacity is utilized by the data stations.

No abstract available.

The scope of this Special Issue is communication in its most general sense, encompassing communication from person to person, between people and machines, and from machine to machine.

The ultimate judges of the quality of reproduced pictures are human observers. In many practical situations it is desirable to have objective measures which closely mirror the performance of the human viewer; today we lack accurate measures. To study this problem, a set of five pictures was used in which distortions of different types were introduced. The resulting series of pictures was rated by viewers by means of an impairment rating test. These ratings were then compared with various objective measures of the error. The objective measure is sought which is most closely monotonically related in a smooth manner to the subjective measure. The average spread of the subjective deviations about a polynomial regression of the subjective measure onto the objective measure is used as the criterion. This technique is used to study both linearly filtered error measures and picture dependent nonlinear weightings of the error. The procedure has enabled a crude quantative model of the human viewer to be determined.

Visibility functions measure the relative visibility of noise added to a picture at those points where some measure of local activity exceeds a given threshold. The functions are obtained from a series of subjective experiments and vary with the content of the picture. Visibility functions have been used to design quantizing characteristics for DPCM coding of monochrome and color signals and for three-dimensional transform coding. We consider an alternative approach to determining the visibility function that obviates the need for repeated picture-dependent subjective tests. The visibility function is assumed to consist of two parts, a picture-dependent component and viewer-dependent component (referred to as the masking function). The visibility function may be approximated by the quotient of a probability density function raised to a power and the masking function. The role of probability is found to be weaker where the viewer has more opportunity to scrutinize the picture.

In the Candy design procedure for DPCM quantizers, the subject is given a specific task to perform. We test the hypothesis that the form of this task influences the results that are obtained. In one experiment the viewers were permitted unlimited time to evaluate noise visibility; in another test using a different testing procedure viewing time was reduced to one second. Visibility functions were obtained from these tests, and quantizers were designed and then evaluated on the basis of impairment ratings of coded pictures. Our results indicate that the design procedure is indeed sensitive to the viewing conditions.

This paper reviews the field of the efficient coding of color television signals. Because this paper is perhaps the first review on this topic, some background is given in the areas of colorimetry, visual perception of color and color television systems. We assume that the reader has some familiarity with luminance encoding techniques. Coding techniques themselves are divided into two broad groups: component coding methods in which each component (usually three) is coded separately, and composite coding methods in which the composite television signal with its "color" modulated subcarrier is processed as a single entity. Both approaches are covered in detail. The field is still growing, pushed primarily by the desire in the television area to find digital coding standards accepted by both broadcasters and carriers and suitable for use with NTSC, PAL and SECAM television systems. We discuss this aspect by comparing composite and component coding methods.

In processing video-telephone signals to reduce bandwidth requirements, lines may occasionally be lost due to buffer overflow or DPCM channel errors. The subjective degradation introduced by replacing deleted lines by averaging or repeating adjacent lines (substitution) has been measured. In the test, both skilled and unskilled observers were asked to add white noise to an unimpaired picture until the quality was equal to the same picture in which a number of lines had been randomly substituted. It was found that the more critical skilled observers deemed a picture, degraded by substituting 1 averaged line per frame, comparable to a signal-to-noise ratio of 38.6 dB, a rating of "definitely noticeable but not objectionable" on a 5-point impairment scale. Futhermore, it was found that repeating lines rather than averaging produced more degradation, equivalent to a 4 dB lower signal-to-noise ratio.

Very simple techniques for estimating the speed of a moving object from a television signal are described. They assume that frame storage is available. One technique requires little more than two threshold circuits and two counters. The accuracy of one technique is better than ± 10 percent, quite sufficient to enable the measure to be used as a control signal for the efficient coding of television type signals.

The properties of the frame-difference signals arising from updated "moving" areas and nonupdated "stationary" areas in conditional replenishment interframe encoders are examined. Expressions for the correlation functions and power density spectra of these signals are developed assuming an infinite image field undergoing uniform linear motion. These expressions are modified so as to include the effect of camera integration. Experimentally obtained power spectra are in good agreement with those predicted. Three-dimensional frame-difference correlation functions are evaluated using a realistic image correlation function. The properties of the frame-difference correlation functions in directions parallel and perpendicular to the direction of movement, and in the temporal direction are delineated. It is shown that the correlation function of the frame-difference signal arising from temporally uncorrelated noise is fundamentally different in updated areas compared with nonupdated areas.

Plateau coding is a method for efficiently coding the chromatic component of color television signals. The signal is divided into regions of approximately constant chromaticity which are transmitted by sending one set of chrominance values for the whole region. Since most changes in chrominance are accompanied by luminance changes, the luminance signal (which must be transmitted quite accurately) is used at the transmitter and receiver to indicate changes in the two chrominance signals; thus no addresses need be sent to define the boundaries of the region at the receiver. Simulations have shown that plateau coding can give pictures of high quality for chrominance bit-rates in the range 0.25-0.5 bits per luminance sample, assuming straightforward pulse-code modulation (PCM) coding of the chrominance amplitudes.

A method is proposed and evaluated for coding a notched scan that jumps back and forth between adjacent scan lines of a normal raster. Alternate points in the line are coded using the normal differential quantizer algorithm while the remaining points use a nonlinear predictor proposed by Graham. The advantage of the algorithm over normal differential quantization is that the prediction strategy is improved while the advantage over other two-dimensional strategies is that the effect of transmission errors is confined to the line pair being processed and does not spread vertically. For coarse coding there is a significant improvement in picture quality when compared with normal differential quantization while for high-quality pictures there is a small reduction in entropy. The effect of transmission errors is investigated and shown to be similar to the effect with normal differential quantization.

The statistical dependence (redundancy) between three differentially coded color components of a video-telephone signal are explored by means of an entropy study. We measure the entropy of the differentially coded color baseband signals both jointly and as three separate signals. The source material consisted of a number of portraittype pictures. The color signal is treated in a format containing a baseband luminance channel and two baseband chrominance channels. Various orthogonal and nonorthogonal transformations, including the Karhunen-Loeve transform, are applied to the chrominance signals to study the effect on the entropy. With a well-chosen format for the color signal, our results show that the redundancy resulting from using a separate encoding of the signals rather than a joint encoding ranges between 0.2 and 0.5 bits. The possible savings accruing from considering the relationship between coded color differential components was only about 1 percent of the total bit rate for most pictures and increased to a maximum of 4 percent for one particular picture.